This invention relates to spinnerette assemblies for forming hollow fibers. It particularly relates to an improved spinnerette for more efficient and precise production of hollow fibers.
It is well known to use various hollow fibers for various applications. For example, hollow fibers are used in carpets, as fill materials for pillows, as insulation materials for blankets and garments, and as membranes for gas separation, blood dialysis, purification of water, and other filtering applications. For membrane applications, the hollow fibers can be bundled together and disposed in a tubular housing to provide a separation device known as a permeator. Ordinarily, the hollow fibers are relatively small, having a diameter on the order of 30 to 1000 micrometers. Accordingly, the apparatus and method for manufacturing hollow fibers must be very precise to be able to control the diameter of the fiber, and the concentricity of the core around the bore.
Numerous spinning assemblies have been devised for the production of hollow fibers. Particularly, devices have been proposed for ensuring uniform supply of the fiber-forming fluid or fluids to the orifices of a spinnerette with the object of producing hollow fibers identical in diameter, composition, and concentricity. These spinnerettes use a means for supplying the bore fluid positioned in the spinning orifice for forming the bore or xe2x80x9chollowxe2x80x9d of the hollow fiber. Usually, a tube or needle is used for this purpose and a gaseous or liquid fluid is injected from the tube, thus forming the bore of the fiber as it is being extruded from the spinnerette orifice. For melt spinning, the nascent fiber can be solidified by cooling in a gaseous or liquid cooling fluid. For solution spinning, the nascent fiber can be solidified by evaporation of the solvent or by contacting the fiber with a solvent-extracting liquid that results in coagulation of the polymer solution(s) to form the fiber wall.
In order to maintain the concentricity of fiber diameter and the bore diameter, other spinnerettes have been provided with members for centering the tube and the bore of the spinnerette plate. For example, U.S. Pat. No. 4,493,629 describes a modular spinnerette assembly fitted with multiple screws threaded through the spinnerette plate to center the tube and orifice of the spinnerette. However, these adjusting screws are unreliable and are prone to error when the spinnerette is disassembled, cleaned and then reassembled.
Typically, spinnerettes of this type are made largely by hand, one at a time. As a result, parts made for one spinnerette will not always fit another spinnerette. When parts are not interchangeable, any damage to one part of the spinnerette assembly may render the entire assembly useless. In assembling or cleaning conventional hollow-fiber spinnerettes, it is very easy to slightly bend the fluid-injection tube or needle, such that it is off center of the spinning orifice. When this happens, the spinnerette cannot be used until repaired.
Yet another problem with existing spinnerette designs is the ability to deliver the polymer fluid uniformly around the tube or needle within the spinnerette. U.S. Pat. No. 5,320,512 discloses a spinneret that has a plurality of discrete material passages formed around the needle to deliver the polymer fluid around the needle. This design enables the uniform delivery of the polymer fluid around the tube or needle within the spinnerette. However, in order to attain concentricity and uniformity in the manufacturing process, the polymer fluid from these individual passages must converge and meld together to form a singular annular flow around the tube or needle as the polymer fluid traverses through the main polymer fluid passage. If complete convergence and melding are not attained, seams may develop down the length of the spun fiber at the interfaces where the individual flows did not fully converge and meld together completely.
Thus, there still remains a need to overcome this problem of interface convergence and melding of the fluid polymer in order to be able to produce a concentric spun hollow fiber with minimal seaming potential.
The present invention provides an improved spinnerette for the production of hollow fibers.
It is an object of the invention to overcome the limitations of conventional spinnerettes.
It is another object of the invention to reduce imperfections in hollow fibers.
It is another object of the invention to extend hollow-fiber production run times.
It is another object of the invention to reduce the time for spinnerette maintenance.
It is another object of the invention to simplify spinnerette fabrication.
To achieve these objects, a first aspect of the invention is a spinnerette assembly for forming a composite hollow fiber comprising:
at least one extrusion orifice formed in said spinnerette assembly;
a hollow needle extending through each said extrusion orifice in a concentric manner to define an annular passage around said needle in said extrusion orifice;
a bore forming fluid passage communicating with an interior of each said needle; and
at least one fiber-forming material passage formed in said spinnerette assembly, wherein each said fiber-forming material passage comprises a fiber-forming material inlet port extending from a surface of said assembly to an interior of said assembly and at least one transverse passage extending from said fiber-forming material port to each said annular passage, wherein a portion of said transverse passage entirely surrounds each said needle in a continuous manner.
A second aspect of the invention is a method of forming a composite hollow fiber comprising the steps of:
delivering a fiber-forming material to each annular passage in the spinnerette assembly, said fiber-forming material entering the spinnerette assembly through one or more fiber-forming material inlet ports and passing through the interior of said assembly to a transverse passage, a portion of said transverse passage entirely surrounding each needle in a continuous manner, and through an annular passage in communication with an extrusion orifice, and
injecting a bore forming fluid into each needle to thereby provide a fiber comprising a bore forming fluid, and a fiber-forming material as it exits the spinnerette assembly through the extrusion orifice,
optionally passing the nascent extruded hollow fiber through an air gap, and
solidifying the hollow fiber by cooling, solvent evaporation, or solvent extraction.